This invention relates to drilling, drill bits and abrasive elements for use in such bits.
Rotary drills comprise a rotatable crown having one end threaded for engagement in the drill rod, stringer or adaptor coupling, and a working portion or cutting face at the other end. The working portion comprises a plurality of cutting elements firmly held in a suitable bonding matrix. The bonding matrix may contain an alloy such as bronze cementing together hard particles such as WC, Fe, or W.
The cutting elements may be made of a variety of hard material such as diamond, cemented carbide and abrasive compacts.
Abrasive compacts, as is known in the art, consist essentially of a mass of abrasive particles present in an amount of at least 70 percent, preferably 80 to 90 percent by volume, of the compact bonded into a hard conglomerate. Compacts are polycrystalline masses containing a substantial amount of direct particle-to-particle bonding. The abrasive particles of compacts are invariably ultra-hard abrasives such as diamond and cubic boron nitride. Diamond compacts are also known in the art as polycrystalline diamond or PCD.
Diamond compacts which are thermally stable at temperatures above 700.degree. C. are known in the art and are used, for example, as the cutting elements in rotary drills. Examples of such compacts are described in U.S. Pat. Nos. 4,534,773, 4,793,828 and 4,224,380. Such cutting elements have generally been provided in the form of cubes or equilateral triangles which are suitably mounted in the cutting face of the rotatable crown of a drill so as to present a cutting point or edge.
European Patent Publication No. 0156235 describes and claims a diamond cutter insert for use in a drill bit which comprises a plurality of thermally stable polycrystalline diamond cutting elements each characterised by a longitudinal axis and held in a matrix material in such manner that the longitudinal axes of the elements are generally mutually parallel. The cutter insert may be mounted on the end of a stud for insertion into a drill bit body. Alternatively, the cutter insert may be bonded directly into the cutting face of a drill bit. The individual polycrystalline diamond cutting elements are said to be capable of having a length of up to 10 mm.
European Pat. No. 0101096 describes a method of producing a plurality of inserts suitable for drills or drill bits including the steps of providing a disc-shaped abrasive compact and severing the compact along planes which are transverse to the flat surfaces of the disc to produce the inserts.
U.S. Pat. No. 4,190,126 describes a rotary abrasive drilling bit comprising a plurality of cutting elements held in a bonding matrix in a working face of the bit, each element comprising a stick-like body of cemented tungsten carbide which presents a curved cutting edge. The drill bit is said to be useful in drilling rock which is of relatively soft formation or semi-hard formation. The drill bit would not be suitable for drilling hard rock formations.